Field of the Invention
This invention relates to improvements in whole crude oil processing, and in particular to an improved method for the demulsification of whole crude oil in a gas-oil separation plant.
Description of Related Art
Crude oil typically contains varying quantities of gas, water and solids based on various well known factors. Water injection processes, in which water is injected into the reservoir to increase pressure and stimulate production, particularly in mature oil fields, increases the water cut, or percentage of water in the produced crude oil. Oil can be present in water as free-oil, an emulsion, and/or dissolved states of varying proportions. “Free-oil” commonly refers to oil droplets of 150 microns or larger which will float immediately to the surface due to their large size and relatively rapid rise velocity. An emulsion is a stable dispersion of oil in water and is formed due to the relatively small diameters of the oil droplets.
Gas oil separation plants (GOSPs) are well known and are used to separate gas, water and oil in order to produce dry crude oil as the end product. Higher water cuts and tight emulsions of wet oil from reservoirs increase the difficulty of and requisite time for the separation in the GOSP. As the water cut increases, the retention time within the separation equipment is increased to cope with the excess water, and, as a result, the rate of oil production is reduced and the GOSP becomes a bottleneck in the oil production.
Oil droplet size distribution is an important factor impacting the design of oil-water separators. Costs associated with treating, handling and disposing of this water increases over time, as separation efficiency is low. The settling velocity (Vt), which measures coarse separation of oil and water, depends on the magnitude of the difference in densities of the two immiscible liquids. The settling velocity function according to Stoke's law is
      V    t    =                    gD        2            ⁡              (                              d            o                    -                      d            w                          )                    18      ⁢      μ                      where, g=gravitational acceleration (m/sec2);        D=diameter of a globule (m);        dw=density of water (kg/m3);        do=density of a globule (kg/m3); and        μ=absolute viscosity (kg/m·sec).The same relationship governs the rising of light liquid droplets in a heavier liquids, in which Vt is a negative value.        
Small oil droplets are more difficult to separate. According to Stoke's law, decreased droplet size results in lower rising velocities. A prerequisite for efficient separation is, therefore, that oil droplets coalesce, i.e., become larger and rise more rapidly.
Water discharge regulations have become more stringent, and compliance in an economical and efficacious manner presents an ongoing challenge to the industry. While several widely accepted techniques exist for removing oil from water, there are limitations, including the oil removal efficiency, i.e., final oil concentration in the treated water, and the oil droplet size for which a selected technique is optimized. Often two to three types of oil-water separation technologies are employed to treat the produced water to the desired lower hydrocarbon concentrations. In a mature oilfield, e.g., one producing oil having a water cut of greater than 30%, the economics of the well change significantly. Accordingly, design characteristics of a free-water knockout (FWKO) should be changed or else the FWKO becomes a bottleneck due to the excess water in the influent crude.
GOSPs are typically designed and constructed to handle oil production from one or more wells situated in one or more reservoirs, the crude oil sources being pooled for processing. The main objective of a GOSP is to increase the flowability and to produce dry crude oil as an end product, e.g., for loading into tankers or for passage through pipelines to the refineries.
In general, a GOSP is a continuous separation process that commonly includes a two-stage or three-stage oil-gas separation facility. Unit operations include a dehydrator unit, a desalting unit, a water/oil separation vessel (WOSEP), a stabilizer column, a high pressure production trap (HPPT) and a low pressure production trap (LPPT). In addition, the GOSP can include boilers, condensers, separation pumps, heat exchangers, mixing valves for addition of demulsification chemicals, skimmers for stabilizing the emulsion, recycle pumps, level valves, relay valves, and control system components such as one or more sensors operatively coupled to a computerized controller or an operator notification system.
Referring to the schematic diagram of FIG. 1, a typical single train GOSP system 10 of the prior art includes an LPPT unit 31, a LPPT unit 41, a wet crude oil holding tank 49, a dehydrator unit 51, a desalter unit 61, a water/oil separator vessel 71, a wastewater vessel 72, a stabilizer column 81, a reboiler 82 and a dry crude oil vessel 91.
A wet crude oil or a tight emulsion crude oil stream 30 from a well pool enters the HPPT unit 31 where crude oil is separated into a gas discharge stream 32, a water discharge stream 33 that is discharged for collection to the water/oil separator vessel 71, and a wet crude oil stream 34. Wet crude oil stream 34 from the HPPT unit 31 is passed to the LPPT unit 41 where the contents are separated into a gas discharge stream 42, a water discharge stream 43 which is discharged for collection to the water/oil separator vessel 71, and a wet crude oil stream 44 which is passed to a wet crude oil holding tank 49.
Wet crude oil stream 48 is pumped from wet crude oil holding tank 49 and is conveyed to a dehydrator unit 51 for further water/oil separation. A water stream 53 is discharged for collection in water/oil separator vessel 71, and a crude oil stream 52 is conveyed to a desalter unit 61. Wet crude oil is washed in desalter unit 61 with aquifer water (not shown), the treated wet crude oil stream 62 is passed to a stabilizer column 81, and a water stream 63 is discharged for collection in water/oil separator vessel 71.
The stabilizer column 81 has a number of trays (e.g., up to sixteen), whereby crude oil flows down over each tray until it reaches a draw-off tray. A reboiler 82 heats dry crude oil from the draw-off tray and returns it to the stabilizer column 81. Light components in the crude oil vaporize and rise through the stabilizer trays. Hydrogen sulfide and light hydrocarbons are removed as a gas stream 84, and a dry crude oil stream 92 is discharged and collected in a dry crude oil vessel 91.
Water/oil separator vessel 71 collects water from streams 33, 43, 53 and 63, and separates oil from the collected water using, e.g., centrifugal pumps. Wastewater is discharged to a wastewater vessel 72 and extracted oil is conveyed to the wet crude oil holding tank 49.
In general, the HPPT unit 31 operates at a pressure of from about 100 pounds-force per square inch gauge (PSIG) to about 200 PSIG and a temperature of from about 50° C. to about 80° C. LPPT unit 41 operates at a pressure of from about 30 PSIG to about 70 PSIG and a temperature of from about 35° C. to about 80° C.
A GOSP is generally designed to treat water cuts in the range of about 30% to about 40% by gravimetric separation. The ultimate goal of a GOSP is to reduce the content of contaminant to a suitable level, e.g., less than 0.2% bottoms, sediment and water (BS&W), and lower concentration of dissolved hydrogen sulfide in order to meet crude oil specifications.
Tight emulsions of oil in water (or water in oil) occur naturally, during transportation of crude oil from the wells to the GOSP, and within the GOSP. The emulsion level in wet crude increases due to agitation and mixing, particularly when production from the wells is enhanced by injected water. In addition, excess water increases the load in the dehydration and desalting units, the time for gravimetric separation, and the requisite quantity of chemical additives.
Tight emulsions can be formed by mechanical mixing and/or chemical action. Chemically-created emulsions are generally due to addition of stabilizers in the reservoir formation. Mechanically-created emulsions are caused by pumping, large pressure drops through chokes, control valves, and other mixing operations. These mechanical forces also impact droplet size. For example, passing fluids through choke valves (from high pressure regions to lower pressure regions) may cause a reduction in droplet size. The mechanical shearing forces can create a high proportion of dispersed oil droplets of 10 μm and less.
Tight emulsions become increasingly difficult to remove, especially when they contain about 1%-4% water in the crude oil. For very tight emulsions, dehydration and desalter units require additional chemicals and an increase in recycling/pumping to remove the water from the emulsions. However, incorporation of chemical additives can be inefficient due to the low retention time and inefficient mixing.
Accordingly, the long-standing problem addressed by the present invention is how to improve whole crude oil processing to increase the crude oil flowability in a GOSP, and in particular, to improve the demulsification of whole crude oil in a GOSP.